Humidifying apparatus, lithographic apparatus and humidifying method

ABSTRACT

A humidifying apparatus is disclosed in which gas is provided to a first side of a membrane and liquid to a second side of the same membrane. The membrane is non-porous to the liquid but porous to vapor of the liquid and is liquidphilic to said liquid.

RELATED APPLICATIONS

This application is a continuation of co-pending Great Britain PCTApplication No. PCT/GB2007/00278 filed Jan. 26, 2007, the entirecontents of which is hereby incorporated by reference.

FIELD

The present invention relates to a humidifying apparatus and ahumidifying method for use in combination with a lithographic apparatusand a method for manufacturing a device.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see for example U.S. Pat. No. 4,509,852) means thatthere is a large body of liquid that must be accelerated during ascanning exposure. This requires additional or more powerful motors andturbulence in the liquid may lead to undesirable and unpredictableeffects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate using a liquidconfinement system (the substrate generally has a larger surface areathan the final element of the projection system). One way which has beenproposed to arrange for this is disclosed in WO 99/49504. As illustratedin FIGS. 2 and 3, liquid is supplied by at least one inlet IN onto thesubstrate, preferably along the direction of movement of the substraterelative to the final element, and is removed by at least one outlet OUTafter having passed under the projection system. That is, as thesubstrate is scanned beneath the element in a −X direction, liquid issupplied at the +X side of the element and taken up at the −X side. FIG.2 shows the arrangement schematically in which liquid is supplied viainlet IN and is taken up on the other side of the element by outlet OUTwhich is connected to a low pressure source. In the illustration of FIG.2 the liquid is supplied along the direction of movement of thesubstrate relative to the final element, though this does not need to bethe case. Various orientations and numbers of in- and out-letspositioned around the final element are possible, one example isillustrated in FIG. 3 in which four sets of an inlet with an outlet oneither side are provided in a regular pattern around the final element.

Another solution which has been proposed is to provide the liquid supplysystem with a seal member which extends along at least a part of aboundary of the space between the final element of the projection systemand the substrate table. Such a solution is illustrated in FIG. 4. Theseal member is substantially stationary relative to the projectionsystem in the XY plane though there may be some relative movement in theZ direction (in the direction of the optical axis). A seal is formedbetween the seal member and the surface of the substrate. Preferably theseal is a contactless seal such as a gas seal to confine the liquid.Such as system with a gas seal is disclosed in EP-A-1,420,298 andillustrated in FIG. 5.

In EP-A-1,420,300 the idea of a twin or dual stage immersion lithographyapparatus is disclosed. Such an apparatus is provided with two stagesfor supporting the substrate. Leveling measurements are carried out witha stage at a first position, without immersion liquid, and exposure iscarried out with a stage at a second position, where immersion liquid ispresent. Alternatively, the apparatus has only one stage.

Many of the modules used in an immersion lithographic apparatus requirethe use of a purge gas. For example, a gas might be needed in a liquidconfinement system or might be needed in other systems such as dryingsystems. In order to reduce cooling effects due to evaporation by use ofa purge gas (which can have quite a high flow rate) it is useful to usehumidified gas. Currently humidified gas which has a relative humidityof 45-85% is used. In future systems it will be necessary to usehumidified gas with a humidity level which is higher, up to 97% relativehumidity. The use of a high humidity gas reduces evaporation. Anotherarea where high humidity gas is required is in use in so called “allwet” immersion systems in which the whole top surface of the substrateis submerged by water during imaging of the substrate. In order toprevent deleterious temperature variations due to evaporation of thewater covering the substrate a high humidity gas is purged over thewater surface so that the air above the water surface is close tosaturation thereby eliminating evaporation. If it were not for theprovision of this purging gas, a large amount of water would be neededin order to keep the temperature stable at normal evaporation rates.

SUMMARY

It is desirable to provide a method and apparatus for the humidifying ofgas.

According to an aspect of the invention, there is provided a humidifyingapparatus comprising: a membrane; a first conduit for guiding gas to oneside of said membrane; a second conduit for guiding liquid to the otherside of said membrane; wherein said membrane has a surface which isliquidphilic to said liquid.

According to an aspect of the invention, there is provided a humidifyingapparatus comprising: a membrane; a liquid temperature conditioner forconditioning the temperature of a liquid; a liquid conduit for guidingliquid from said liquid temperature conditioner to one side of saidmembrane; and a gas conduit for guiding gas to the other side of saidmembrane; wherein the temperature of said gas on the other side of saidmembrane is influenced by the temperature of liquid on the one side ofsaid membrane.

According to an aspect of the invention, there is provided a method ofhumidifying gas comprising: providing a liquid on one side of a membranewhich membrane is liquidphilic to said liquid; and providing a gas to behumidified on the other side of said membrane.

According to an aspect of the invention, there is provided a method ofhumidifying gas comprising: controlling the temperature of a liquid andproviding that liquid to one side of a membrane; and providing gas to behumidified to the other side of said membrane, wherein the temperatureof gas on the other side of said membrane is influenced by thetemperature of liquid on the one side of the membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system used in a prior artlithographic projection apparatus;

FIG. 4 depicts a liquid supply system according to another prior artlithographic projection apparatus;

FIG. 5 depicts, in cross-section, a liquid supply system of an immersionlithographic projection apparatus;

FIG. 6 depicts schematically a circuit diagram of an example of a purgegas supply system according to the invention;

FIGS. 7 a and b depict schematically a humidifying apparatus accordingto the present invention; and

FIG. 8 illustrates a second embodiment of a humidifying apparatusaccording to the present invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g. UV radiation or DUV radiation).    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PS configured to project a pattern imparted to the radiation        beam B by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the mask MA with respect to the pathof the radiation beam B, e.g. after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

The use of higher and higher flow rates of gas in immersion lithographyapparatus, particularly in liquid confinement systems but also in othersystems such as drying stations can lead to fast evaporation ofimmersion liquid from, for example, the substrate. This high evaporationrate leads to cooling and deleteriously effects the performance of theapparatus. Another area where high humidity gas is required is in use inso called “all wet” immersion systems in which the whole top surface ofthe substrate is submerged by water during imaging of the substrate. Inorder to prevent deleterious temperature variations due to evaporationof the water covering the substrate a high humidity gas is purged overthe water surface so that the air above the water surface is close tosaturation thereby eliminating evaporation. If it were not for theprovision of this purging gas, a large amount of water would be neededin order to keep the temperature stable at normal evaporation rates.Also, increasing flow rates of humidified gas are necessary to minimizethe disturbance for all optical sensors (examples: level sensor purging,encoder sensor purging etc.). These increased flows therefore require amore efficient humidifying apparatus.

FIG. 6 illustrates a general purge gas supply system which might be usedin an immersion lithographic apparatus. Such a system is described indetail in US 2005/0051739 and reference is made to that document.

FIG. 6 shows a practical example of a purge gas supply system 100.However, a similar system as shown in FIG. 6 may likewise be utilized inconditioning gas used in gas bearings in e.g. an immersion lithographyapparatus. In the example of FIG. 6, a purge gas inlet 110 is connectedto a, not shown, purge gas supply apparatus which supplies a dry gaswhich is substantially without moisture, such as, for example, apressurised gas supply circuit, a cylinder with compressed dry air orotherwise. The dry gas is fed through the purge gas mixture generator120. In the purge gas mixture generator 120 the dry gas is purifiedfurther, as is explained below in more detail. Further, the purge gasmixture generator 120 includes a humidifying apparatus 150 which adds amoisture to the dry gas, for some of the purge gas outlets 130-132. Inthe example of FIG. 6, the humidifying apparatus 150 is connected asingle purge gas outlet 130. The other purge gas outlets 131, 132 arenot connected to the humidifying apparatus 150. Thus, at the purge gasoutlet 130, a purge gas mixture including the purge gas and moisture ispresented, whereas at the other purge gas outlets 131,132 only the drypurge gas is presented. Thereby the purge gas mixture may be providedonly near surfaces provided with chemicals which require a moisture,such as the substrate table WT in the example of FIG. 1, whereas otherparts of the lithographic projection apparatus 1 can be provided with a‘dry’ purge gas, i.e, without moisture. However, the main reason for useof humidified purge gas is not to disturb measurement systems (inparticular for non immersion lithographic projection apparatus).

Furthermore, because the moisture is added to a purge gas, properties ofthe purge gas mixture, such as the relative humidity or purity of themoisture, can be controlled with a good accuracy. Also, because of themoisturizer the system is flexible, because the amount of moisturepresent in the purge gas mixture may easily be adjusted by adding moreor less moisture to the purge gas.

The purge gas mixture generator 120 in the example of FIG. 6 includes,in a flow direction and that order: a purifier apparatus 128, a flowmeter 127, a valve 125, a reducer 129, a heat exchanger 126 and ahumidifying apparatus 150.

In the example of FIG. 6, compressed dry air (CDA) from a, not shown,CDA source is supplied to the purifier apparatus 128 via the purge gasinlet 110. The CDA is purified by the purifier 128. The purifier 128includes two parallel flow branches 128A, 128B each including, in theflow direction and that order: an automatic valve 1281, 1282 and aregenerable purifier device 1283,1284. The regenerable purifier devices1283,1284 are each provided with a heating element to heat and therebyregenerate the respective purifier device 1283, 1284. The flow branchesare connected downstream of the purifier devices 1283, 1284 to ashut-off valve 1285 which is controlled by a purity sensor 1286.

Because of the regenerable purifiers, the system can be used for a longtime period by regenerating the purifiers in case they become saturatedwith the compounds removed from the purge gas. The regenerable purifiersmay be of any suitable type, such as for example a, known as such,regenerable filter which removes contaminating compounds or particlesout of a gas by means of a physical process, such as adsorption,catalysis or otherwise, as opposed to the, non regenerable, chemicalprocesses occurring in a charcoal filter, for example. In general, aregenerable purifier does not contain organic material and theregenerable purifiers may for example contain a material suitable forphysical binding a contaminant of the purge gas, such as for example:metals, zeolite, titanium oxides, gallium or palladium compounds, orotherwise.

In the example of FIG. 6, the purifier devices 1283, 1284 arealternately put in a purifying state in which the CDA is purified and aregenerating state. In the regenerating state the purifier device isregenerated by means of the respective heating element. Thus, forexample, while the purifier device 1283 purifies the CDA, the purifierdevice 1284 is regenerated. The purifier 128 can thus operatecontinuously while maintaining a constant level of purification.

The automatic valves 1281, 1282 are operated in correspondence with theoperation of the corresponding purifier device 1283, 1284. Thus, when apurifier device 1283, 1284 is regenerated, the corresponding valve 1281,1282 is closed, while when a purifier device 1283, 1284 is used topurify, the corresponding valve is open.

The purified CDA is fed through the shut-off valve 1285 which iscontrolled by the purity sensor 1286, which is known per se and for thesake of brevity is not described in further detail. The purity sensor1286 automatically closes the shut-off valve 1285 when the purity of thepurified CDA is below a predetermined threshold value. Thus,contamination of the lithographic projection apparatus 1 with a purgegas with insufficient purity levels is prevented automatically.

The flow of purified CDA can be monitored via the flow meter 127. Viathe valve 125 the flow can be shut-off manually. The reducer 129provides a stable pressure at the outlet of the reducer, thus a stablepurge gas pressure is provided to restrictions (via the heat exchanger126).

The heat exchanger 126 provides a constant purified CDA temperature. Theheat exchanger 126 extracts or adds heat to the purified CDA in order toachieve a gas temperature which is suitable for the specificimplementation. In a lithographic projection apparatus, for example,stable processing conditions are required and the heat exchanger maythus stabilize the temperature of the purified CDA to have a gastemperature which is constant over time. Suitable conditions for thepurge gas at the purge gas outlets, for example, are found to be: a flowof 50-60 standard liters per minute, (for immersion systems typically60-120 standard liters per minute when the gas is used for liquidconfinement, 100-120 standard liters per minute when used to preventevaporation of water covering the substrate and/or a temperature of thepurge gas of about 22 degrees Celsius and/or a relative humidity in therange of 30-99%. However, the invention is not limited to theseconditions and other values for these parameters may likewise be used ina system according to the invention.

The heat exchanger 126 is connected via restrictions 143-145 to thepurge gas outlets 130-132. The restrictions 143-145 limit the gas flow,such that at each of the purge gas outlets 130-132 a desired, fixedpurge gas flow and pressure is obtained. A suitable value for the purgegas pressure at the purge gas outlets is for example 100 mbar. It islikewise possible to use adjustable restrictions to provide anadjustable gas flow at each of the purge gas outlets 130-132.

The humidifying apparatus 150 is connected downstream from the heatexchanger between the restriction 143 and the purge gas outlet 130. Thepurge gas outlet 130 is provided in the example of FIGS. 1 and 2 nearthe substrate table WT. The humidifying apparatus 150 adds a moisture tothe purified CDA and thus provides a purge gas mixture to the outlet130. In this example, only at a single outlet a purge gas mixture isdischarged. However, it is likewise possible to discharge a purge gasmixture to two or more purge gas outlets, for example by connecting amultiple of purge gas outlets to separate moisturizers or connecting twoor more outlets to the same moisturizer. It is likewise possible toprovide a moisturizer at a different position in the purge gas mixturegenerator than is shown in FIG. 6. For example, the humidifyingapparatus 150 may be placed between the purge gas mixture generator 120and the valve 143 instead of between the valve 143 and the purge gasoutlet 130. The humidifying apparatus 150 may operate as a restrictionas well and if so desired, the restriction 143 connected to thehumidifying apparatus 150 may be omitted.

In an alternative embodiment of a purge gas supply system according tothe invention, an additional heat exchanger (not shown) is provided atthe purge gas outlet 130 for a better temperature control of the purgegas mixture.

FIGS. 7 and 8 illustrate humidifying apparatus 150 according to thepresent invention which may be used with the purge gas supply system ofFIG. 6.

The basic arrangement of the humidifying apparatus 150 is illustrated inFIG. 7 a and a more complex structure is illustrated in FIG. 7 b whichworks on the same principles. In FIG. 7 a the humidifying apparatuscomprises a membrane 200 which behaves as if non-porous to the liquidwith which the gas is to be humidified (usually (ultra pure) water) andporous to the vapour of that liquid. It should be possible to pressurizethe water side of the membrane (though this is not necessary with ahydrophilic membrane as described below) and no water should passthrough the membrane. One can also pressurize the gas side (at leastusing air or its components like nitrogen, oxygen etc.) and no gas willpass through the membrane either. The liquid wets the membrane materialand then evaporates. The membrane allows only vapour of the liquid topass and the liquid molecules can leave the membrane as they go intogas. A little gas may dissolve into the liquid from the gas side of themembrane but no bubbles form in the liquid. Thus, the membrane can beseen as porous to vapour of the liquid.

A first conduit 210 guides a gas to be humidified to one side of themembrane 200 and a second conduit 220 guides the liquid to humidify thegas to the other side of the membrane 200. As the gas is present onone-side of the membrane 200 and the liquid on the other side of themembrane 200 vapour from the liquid will pass through the membrane 200and humidify the gas. Preferably a flow of gas is provided past themembrane 200 so that a third conduit 230 is provided for guiding the gaswhich has been humidified away from the membrane 200 and a fourthconduit 240 is provided for guiding liquid away from the other side ofthe membrane 200.

A similar humidifying apparatus to this is disclosed in WO 2005/010619and much of what is said in that application is applicable here inparticular regarding the shape of the membrane, how it is connected etc.However, the major difference to what is disclosed in WO 2005/010619 isthat the membrane of the present invention is provided with aliquidphilic surface. In other words, the liquid on the one side of themembrane 200 has a contact angle with the membrane of less than 90°,preferably less than 70°, more preferably less than 60°, yet morepreferably less than 50° and most preferably less than 30° or even lessthan 20°.

One suitable class of material for use as the membrane are polymerizedfluorinated sulfonic acid copolymers which are synthetic ionic polymers.The sulfonic acid groups are chemically active, but they are fixedwithin the polymer matrix. One such material has the following chemicalformula:

The hydrophilic nature of the membrane could be provided, for example,through a coating (on one or both sides of the membrane) and/or throughapplying an electrical potential to the membrane, for example. Thesetypes of membranes have previously been used in steam purificationsystems such as those sold by Rasirc of San Diego, Calif., US under thetrade name Intaeger.

The advantage of such a membrane is that a low pressure of liquid can beused on the liquid side of the membrane and there is better efficiencyof gas side evaporation of the liquid i.e. there is greater masstransfer across the membrane. Furthermore, for hydrophilic membranes, itis not necessary to pressurize the water so that the actual hardware hasconsiderably lower pressure drop off (factor of 10) compared tohydrophobic membranes. This characteristic is what makes the presentinvention feasible.

It is advantageous to maximize the surface area of membrane and apreferred embodiment is where the membrane is a hollow fibre with theliquid passing through the inside of the hollow fibre and the gaspassing over the outside of the hollow fibre (though vice versa could betrue also). One such embodiment is illustrated in FIG. 7 b in which theliquid is provided through hollow fibre 2000 which is comprised of themembrane 200. Only one fibre is illustrated in FIG. 7 b but of coursethe second conduit 220 could be connected to several fibres in parallel.

In the embodiment of FIG. 7 b the gas enters a housing 250 whichsurrounds the hollow fibres 2000 and is passed over the fibres afterbeing guided by conduit 210 into the housing and then, once humidified,the gas is guided out of the housing by third conduit 230.

In order to provide the flow of gas and flow of liquid, a liquidprovider and gas provider are necessary. These could take the form, forexample, of a pump providing the liquid and a compressed gas source.

FIG. 8 illustrates a second embodiment of humidifying apparatus in whichthe temperature of the gas leaving the humidifying apparatus 150 iscontrolled through careful control of the temperature of the liquidentering the humidifying apparatus 150 through second conduit 220. Tothis end the liquid is conditioned by a liquid conditioning apparatus1200 prior to being supplied to the second conduit 220. The conditioningapparatus 1200 brings the temperature of the liquid to a desiredtemperature based on the input or output temperature of gas into or outof the humidifying apparatus 150. Therefore, the heat exchanger 126 ofthe purge gas supply system is not necessary as this function isautomatically performed by the humidifying apparatus 150. The fourthconduit 240 may of course be provided back to the temperatureconditioner 1200 so that the liquid is re-used. Alternatively, a heatersimilar to heater 126 of the purge gas supply system could be usedeither to heat the gas upstream or downstream of the membrane. Heatingis required because of the loss in temperature on evaporation of theliquid.

Furthermore, the humidifying apparatus in FIG. 8 may be comprised of twomodules. A local module comprises parts 150 and 1200 and these can bepresent only where a very high relative humidity of gas is required.Thus, the first conduit 210 can be provided with pre-humidified gaswhich has been humidified by apparatus 1150. This arrangement isparticularly good because there can be a large pressure drop from wherethe gas is humidified to where it is actually used. If there is pressuredrop, the humidity of a gas also decreases. Thus, providing the gas withhigh relative humidity locally using parts 150 and 1200 it is possibleto place the module much closer to the exit for the gas so that thepressure drop between the humidifier and the exit hole is minimized.Furthermore, there are only a few modules which require extremely highhumidity (97%) so that this way the energy used in achieving thathumidity is decreased.

Humidifying apparatus 1150 may be any type of humidifying apparatusincluding, but not limited to a bubbling humidifying apparatus such asthat disclosed in US 2005/0051739, or an apparatus such as thatdisclosed in WO 2005/010619 (i.e. a humidifier with a liquidphobicmembrane) or may be a humidifier in accordance with the presentinvention and as illustrated in FIG. 7. The pre-humidifier provideshumidified air to all modules (at about 40-45% relative humidity or atleast less than 97% relative humidity) at a flow of about 400 liters perminute.

The humidifying apparatus 1150 may be common to the whole of thelithographic apparatus and only part of the liquid provided by thatapparatus is supplied to the humidifying apparatus 150, as required.

Typically the humidifying apparatus 1150 would provide gas of relativehumidity of between 40 and 60% if the humidified air of the overallmodule is to be used in clean rooms. This could be increased to up to 80or 90% (which is 60% after expansion at typical pressure drop betweenthe modules). The gas then leaving the other humidifying apparatus 150would have a relative humidity of at least 90%, preferably at least 97%.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The present invention can be applied to any immersion lithographyapparatus, in particular, but not exclusively, those types mentionedabove.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A humidifying apparatus comprising: a membrane; a first conduitconfigured to flow gas onto one side of said membrane; a second conduitconfigured to flow liquid onto the other side of said membrane; and apre-humidifier apparatus, other than the membrane, configured topre-humidify the gas for said one side upstream of said first conduit,wherein said membrane has a surface which is liquidphilic to saidliquid, and, in use, behaves in a non-porous manner with regard to saidliquid such that the liquid does not enter the membrane, and in a porousmanner with regard to the vapour of said liquid.
 2. The humidifyingapparatus according to claim 1, wherein said surface is hydrophilic. 3.The humidifying apparatus according to claim 1, wherein said liquid hasa contact angle with said surface of less than 90°, or less than 70°, orless than 60°, or less than 50°, or less than 30°.
 4. The humidifyingapparatus according to claim 1, further comprising a liquid providerconfigured to provide liquid to the second conduit.
 5. The humidifyingapparatus according to claim 1, further comprising a gas providerconfigured to provide gas to said first conduit.
 6. The humidifyingapparatus according to claim 1, further comprising a third conduitconfigured to guide gas away from said one side.
 7. The humidifyingapparatus of claim 1, further comprising a fourth conduit configured toguide liquid away from said other side.
 8. The humidifying apparatus ofclaim 1, wherein said membrane is in the form of a tube, said one sideof said membrane being the inside of said tube.
 9. The humidifyingapparatus of claim 1, wherein said membrane is comprised of afluorinated sulfonic copolymer.
 10. The humidifying apparatus of claim1, further comprising a liquid temperature conditioner configured tocondition the temperature of said liquid upstream of said second conduitand/or configured to condition the temperature of said gas upstreamand/or downstream of said membrane.
 11. The humidifying apparatus ofclaim 10, further comprising a controller configured to control theliquid temperature conditioner to change the temperature to which saidliquid or gas is conditioned and/or the flow rate of said liquid or gas.12. The humidifying apparatus of claim 11, wherein the controller isconfigured to control on the basis of gas temperature entering orleaving the one side of said membrane.
 13. An immersion lithographicapparatus comprising a humidifying apparatus of claim
 1. 14. Theimmersion lithographic apparatus of claim 13, further comprising apurging system configured to blow humidified gas from said thirdconduit, said purging system is at least one of a liquid confinementsystem, a drying station, or an evaporation reducing system.
 15. Thehumidifying apparatus of claim 1, further comprising: a liquidtemperature conditioner configured to condition the temperature of aliquid, wherein the second conduit is configured to guide liquid fromsaid liquid temperature conditioner to the other side of said membrane,and wherein the temperature of said gas on the one side of said membraneis influenced by the temperature of liquid on the other side of saidmembrane.
 16. The humidifying apparatus of claim 15, wherein said liquidtemperature conditioner is configured to receive control signalsindicative of a desired temperature of said liquid based on an actualand/or desired temperature of said gas.
 17. The humidifying apparatus ofclaim 16, wherein said actual temperature of said gas is the temperatureof gas leaving said one side of said membrane.
 18. A method ofhumidifying gas comprising: flowing a liquid onto one side of a membranewhich membrane is liquidphilic to said liquid, and, in use, behaves in anon-porous manner with regard to said liquid such that the liquid doesnot enter the membrane, and in a porous manner with regard to the vapourof said liquid; and flowing a gas to be humidified onto the other sideof said membrane, wherein said gas provided to said other side of saidmembrane is pre-humidified by an apparatus other than said membraneprior to being provided to said other side of said membrane.
 19. Themethod of claim 18, wherein said liquid provided to one side of saidmembrane has its temperature regulated thereby to condition thetemperature of said gas.
 20. The method of claim 18, wherein said liquidhas a contact angle of less than 90°, or less than 70°, or less than60°, or less than 50°, or less than 30° with said membrane.
 21. Themethod of claim 18, wherein said membrane comprises at least oneelongate pipe within which said gas or said liquid is provided.
 22. Themethod of claim 18, wherein said providing a liquid comprises providinga flow of liquid.
 23. The method of claim 18, wherein said membrane ismade of a fluorinated sulfonic copolymer.
 24. A method of containing aliquid in an immersion lithographic apparatus using a gas humidified bythe method of claim
 18. 25. The method of claim 18, wherein saidproviding a gas comprises providing a flow of gas.
 26. The method ofclaim 18, wherein said liquid is ultra pure water.
 27. The method ofclaim 18, further comprising: controlling the temperature of a liquidand providing that liquid to the one side of a membrane, wherein thetemperature of gas on the other side of said membrane is influenced bythe temperature of liquid on the one side of the membrane.